Cover story: Energy-Efficient Electric Motors

How to get the most out of energy-efficient motors: You have to do more than just look at the motor nameplate to get maximum energy savings. Are you ready for Dec. 19, 2010, the next major U.S. motor efficiency compliance deadline? Those truly interested in increasing efficiency will take a holistic view of overall machine design. Here's how to get inside motor efficiency to maximize savings.

C.G. Masi for Control Engineering

11/15/2010

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Concern for the environment may motivate private individuals and the general public, but it’s a poor reason for making business decisions. Companies buying and installing electric motors have a far better business reason for trying to save electricity— making their operations more cost-efficient. In a rare confluence of factors, reducing electricity usage by installing more energy-efficient electric motors promotes both of these goals. And, it might be a requirement as of Dec. 19, 2010, the next major U.S. motor efficiency compliance deadline.

When one talks about high-efficiency motors, it is important to note that this term really refers to traditionally architected electric motors consisting of armature and field windings. “Induction motors are available in standard-, high- and premium-efficiency models,” said David Hansen, global product manager, Kinetix Motion Control, Rockwell Automation, “whereas permanent magnet motors are not.”

The reason is that the permanent-magnet electric motor architecture is inherently more efficient, since no power is used to establish the stator magnetic field. John Malinowski, senior product manager - ac motors, Baldor Electric Company, pointed out: “ac induction motors have a family of motors that comply with NEMA Premium Efficiency per NEMA MG 1, tables 12-12 and 12-13, and IEC 60034- 30 [standards] for IE3 efficiency.”

For that reason, we limited this discussion to induction motors having stator coils wound on ferromagnetic cores, and save looking at the energy-efficiency characteristics of permanent magnet motors for a later discussion.

“Premium motors,” Malinowski continued, “are built to closer tolerances than older motors, run cooler, have less vibration, are quieter, and last longer.”

However, “high efficiency in today’s ac induction motors,” said Peter Fischbach, industry sector manager, Bosch Rexroth Corp., “is achieved by improving the energy conversion paths and physical properties with new slot and winding geometries, advanced magnet and core materials, and the use of copper rotors for ac induction motors.”

Lamination Material—Core losses are directly influenced by the material properties and quality of the steel used in the stator laminations. In addition, thinner laminations will lead to lower core losses in the stator than thicker laminations.

“The goal in most factory automation and industrial applications,” Fischbach suggesteded, “is efficient use of energy with the highest productivity. Therefore it is essential to analyze, model, and optimize the complete system before investing in individual components, like new motors.”

Malinowski agreed: “Better motors are easy to do as drop-in replacements [for less efficient units] but efficiency gains are limited. Using a 95% efficient motor is good but not when connected to a double-reduction [geartrain] that is 50% to 60% efficient, when a helical or bevel speed reducer may be 90% to 95% efficient.”

Fischbach also concurred: “Higher efficiency is a relative term there since we also have to consider other factors [that] affect the overall system efficiency, like cycle time or product output. For example, a direct drive torque motor with 80% efficiency could save more energy than a 95% efficient servo motor by eliminating inefficient drive train elements like gear boxes—and significantly increase production, too.”

What not to do

“The biggest mistakes,” Fischbach warned, “are made by engineers just focusing on the name plate efficiency of the motor, and expecting similar percentage energy savings in their specific application.

Different motors have different characteristics, which have to be matched to the application to benefit from the investment in a higher efficiency motor. For example, a more expensive premium efficiency ac induction motor will not save much energy if it’s run at partial load or idling for long periods of time.”

Malinowski provided the example of replacing a much older motor with a new premium motor on a centrifugal pump. The impeller would have been sized for the old motor’s speed. However, the new, more efficient, motor likely will run faster under the same load, leading to more overall power use. The system might be more energy-efficient, but the additional work it does may not provide any benefit.

“Designers who are truly interested in increasing efficiency,” advised Hansen, “will not simply look to replace a motor, but will take a holistic view of the overall machine design. Even a perfectly efficient motor, if connected to a poorly designed mechanical system, will not produce significant benefits with respect to energy savings. Any mechanical power transmission device between the motor and the load will have inherent inefficiency. The highest precision helical planetary gear heads, brand new out of the box, are at best 90% to 95% efficient. Worm gear boxes can be as little as 50% to 60% efficient.”

“The ultimate solution in terms of machine efficiency,” he concluded, “would be to eliminate the mechanical power transmission devices altogether through the use of direct drive permanent magnet servo motors.”

Brushless PM Torque Motors - Electric motors come in a rich variety of configurations to suit different purposes. One specialty motor type—known as a direct-drive, permanent-magnet (PM) torque motor—is characterized by a large diameter-to-length ratio and large number of magnetic poles to optimize torque production.

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